[CF-metadata] axis attribute

From: Jonathan Gregory <j.m.gregory>
Date: Sat, 23 Dec 2006 22:17:32 +0000

Dear all

This debate went quiet about a month ago. I'd like to propose the following,
in the light of what was said then.

Requirement: A way to identify the horizontal coordinate variables.

(NB coordinate variables means 1D, in the Unidata sense, as usual in CF.)

Proposal: It can be done using COARDS rules for units in the case of latitude
and longitude. In other cases (such as projection coordinates, rotated latitude
and longitude) the standard name can be used, but that requires recognising
specific names, and standard names are optional anyway. The axis attribute is
also optional but if present it can be used to identify horizontal coordinates
as those having axis="X" or axis="Y". If both X- and Y-axis are identified,
X-Y-up should define a right-handed coordinate system i.e. rotation from the
positive X direction to the positive Y direction is anticlockwise if viewed
from above. The axis attribute is allowed for longitude (X) and latitude (Y)
coordinate variables. The axis attribute is not allowed for auxiliary
coordinate variables. Auxiliary coordinate variables can be identified as
lying in the horizontal surface (e.g. 2D latitude and longitude) by the fact
of their having dimensions which all belong to 1D horizontal coordinate
variables.

Change to current standard: Remove the prohibition on axis attribute for
rotated lat and lon. Introduce the requirement for X-Y-up to be right-handed.
Add a remark about how aux coord vars can be inferred to be horizontal. Make
it clear that axis is not allowed on aux coord vars (but that is probably
already the case, so not a change to the standard).

Cheers

Jonathan
Received on Sat Dec 23 2006 - 15:17:32 GMT